Dual bed catalytic system for the reduction of NOx

ABSTRACT

The present invention relates to a dual bed catalytic system for the reduction of nitrogen oxides comprising silver-deposited alumina (Ag/Al 2 O 3 ) and zeolite (ZSM-5). The present dual bed catalytic system provides significantly improved NOx reducing efficiency as compared to the system using Ag/Al 2 O 3  or ZSM-5.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of Korean Patent Application No. 10-2006-0076274, filed on Aug. 11, 2006, the entire disclosure of which is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a dual bed catalytic system for the reduction of nitrogen oxides, which comprises alumina-supported silver (Ag/Al₂O₃) and zeolite (ZSM-5).

Diesel vehicles, compared with non-diesel vehicles, produce an exhaust gas with a relatively low content of carbon monoxide (CO) and hydrocarbons but with a relatively high content of particulate matters (PM) and nitrogen oxides (NO_(x)). The PM and NO_(x) cause various environmental problems and diseases and thus have been under strict regulation in developed countries.

For example, European countries provides environmental regulations such as EURO IV and V, and the United States provides SULEV and ZEV. Those regulations are so strict that it is difficult to be satisfied only by improving the functions of engines. That is, development of post-treatment techniques as well as the improvement of engine functions is essential to meet the above strict regulations.

Intensive researches have been made to provide PM and/or No_(x) removal techniques. For example, a diesel particulate filter (DPF) system was suggested by Johnson Matthey, Engelhard, Umicore. Also, a diesel particulate NO_(x) reduction (DNPR) system was proposed by Toyota Motor Company to reduce both PM and NOx. The DNPR system, however, has disadvantages in that it is to be operated under overloaded conditions and tends to be poisoned by sulfur contained in fuel.

Also, urea (or ammonia) selective catalytic reduction (SCR) system was proposed. However, this system requires not only apparatuses for transporting, infusing and injecting urea but also infrastructure for supplying urea.

One of the catalytic systems developed to overcome the above problems is the HC-SCR system, in which the reductant of NO_(x) is formed from diesel fuel. For example, Cleaire's Longview and Lonestar developed a HC-SCR catalyst system that can effectively retrofit existing cars. (see http://www.fleetguard.com). The system, however, shows a low catalytic activity.

Ag/Al₂O₃ is a HC-SCR catalyst. The Ag/Al₂O₃ catalyst offers good activity at 350° C. or higher in a system using a long chain hydrocarbon (e.g., n-octane) as a reductant [R. Burch, J. P. Breen, C. J. Hill, B. Krutzsch, B. Konrad, E. Jobson, L. Cider, K. Eranen, F. Klingstedt, L-E. Lindfors, CAPOC 6 Meeting (2003); S. Satokawa, J. Shibata, K.-I. Shimizu, A. Satsuma, T. Hattori, Appl. Catal. B 42 (2003), 179]. The Ag/Al₂O₃ catalyst can be applied to a gasoline lean burn engine or a diesel engine having a secondary fuel injection system [T. Nakatsuji, R. Yasukawa, K. Tabata, K. Ueda, M. Niwa, Appl. Catal. B, 17 (1998) 333]. The catalyst, however, has a problem that the catalytic activity of reducing NO_(x) to N₂ (i.e., deNO_(x) activity) at 300° C. or lower is so much low as not to be industrially applicable.

Thus, there still exists a need for an improved catalyst system.

The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.

SUMMARY OF THE INVENTION

The present inventors have made various efforts to solve the above-described problems and provide a dual bed catalytic system comprising ZSM-5 and Ag/Al₂O₃, which shows an improved deNO_(x) activity at 300° C. or lower as well as at 350° C. or higher.

In a preferred embodiment, the present invention provides a dual bed catalytic system for the reduction of NO_(x), which comprises Ag/Al₂O₃ and ZSM-5.

Preferably, Ag/Al₂O₃ may comprise 1-5 wt % of Ag and 95-99 wt % of Al₂O₃.

Also preferably, Ag/Al₂O₃ and ZSM-5 may be packed.

A suitable form of ZSM-5 may be a copper (Cu) deposited ZSM-5. Preferably, the Cu-deposited ZSM-5 may comprise 0-5 wt % of Cu and 95-100 wt % of ZSM-5.

A preferable ratio of Ag/Al₂O₃ to ZSM-5 is 4:1 to 1:4 based on weight. Suitably, Ag/Al₂O₃ is located at the front part of the present catalytic systems and the ZSM-5 is located at the rear part thereof.

Also suitably, the present systems may comprise a C₆-C₁₆ hydrocarbon compound as a reductant of NO_(x).

It is understood that the term “vehicle” or “vehicular” or other similar term as used herein is inclusive of motor vehicles in general such as passenger automobiles including sports utility vehicles (SUV), buses, trucks, various commercial vehicles, watercraft including a variety of boats and ships, aircraft, and the like. The present systems will be particularly useful with a wide variety of motor vehicles.

Other aspects of the invention are discussed infra.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features of the present invention will be described with reference to certain exemplary embodiments thereof illustrated the attached drawings in which:

FIG. 1 shows the deNO_(x) activity of dual bed catalytic systems each of which includes Ag/γ-Al₂O₃ with Cu/ZSM-5, Cu/Y, Fe/ZSM-5 and H/ZSM-5, respectively, with the packing ratio of 2:1, wherein F stands for front catalyst, R stands for rear catalyst, Ag (2.00) stands for 2 wt % Ag/γ-Al₂O₃, Cu (4.66) stands for 4.66 wt % Cu/ZSM-5, CuY (8.17) stands for 8.17 wt % Cu/Y and Fe (1.96) stands for 1.96 wt % Fe/ZSM-5;

FIG. 2 shows the deNO_(x) activity of dual bed catalytic systems which comprise 3.37 wt % Cu/ZSM-5 and 2 wt % Ag/γ-Al₂O₃, wherein F stands for front catalyst, R stands for rear catalyst, Ag (2.00) stands for 2 wt % Ag/γ-Al₂O₃ and Cu (3.37) stands for 3.37 wt % Cu/ZSM-5;

FIG. 3 shows the deNO_(x) activity of dual bed catalytic systems which comprise 2 wt % Ag/γ-Al₂O₃ (front) and 3.37 wt % Cu/ZSM-5 (rear), wherein F stands for front catalyst, R stands for rear catalyst, Ag (2.00) stands for 2 wt % Ag/γ-Al₂O₃ and Cu (3.37) stands for 3.37 wt % Cu/ZSM-5;

FIG. 4 shows the deNO_(x) activity of dual bed catalytic systems which comprise 2 wt % Ag/γ-Al₂O₃ (front) and 4.66 wt % Cu/ZSM-5 (rear), wherein F stands for front catalyst, R stands for rear catalyst, Ag (2.00) stands for 2 wt % Ag/γ-Al₂O₃ and Cu (4.66) stands for 4.66 wt % Cu/ZSM-5;

FIG. 5 shows the deNO_(x) activity of dual bed catalytic systems which comprise 2 wt % Ag/γ-Al₂O₃ (front) and x wt % Cu/ZSM-5 (rear), wherein F stands for front catalyst, R stands for rear catalyst, Ag (2.00) stands for 2 wt % Ag/γ-Al₂O₃ and Cu (x) stands for x wt % Cu/ZSM-5; and

FIG. 6 shows the deNO_(x) activity of dual bed catalytic systems which comprise x wt % Ag/γ-Al₂O₃ (front) and 4.66 wt % Cu/ZSM-5 (rear), wherein F stands for front catalyst, R stands for rear catalyst, Ag (x) stands for x wt % Ag/γ-Al₂O₃ and Cu (4.66) stands for 4.66 wt % Cu/ZSM-5.

DETAILED DESCRIPTION

Reference will now be made in detail to the preferred embodiments of the present invention.

As discussed above, in a preferred aspect, the present invention provides dual bed catalytic systems for the reduction of NO_(x) comprising Ag/Al₂O₃ and ZSM-5. The present systems have significantly improved NO_(x) reducing efficiency compared with conventional systems comprising Ag/Al₂O₃ only or ZSM-5 only. More particularly, the present systems have excellent reducing activity at a low temperature at which NO_(x) reduction is almost impossible with the conventional system using Ag/Al₂O₃ only.

A preferred system of the present invention may have a packing structure of Ag/Al₂O₃ and ZSM-5. Catalytic activity of a packed dual bed catalytic system may vary depending on the packing sequence of catalysts, packing ratio, Ag content of Ag/Al₂O₃, the kind and content of metal ions deposited in ZSM-5, etc. Thus, a customized catalyst system may be prepared depending on applications.

More specifically, deNO_(x) activity varies depending on Ag content of Ag/Al₂O₃. A preferable Ag content is 1 to 5 wt %. More preferable Ag content is 2 to 3 wt %. If the Ag content is below 1 wt %, deNO_(x) activity becomes poor. If the content exceeds 5 wt %, deNOx activity does not improve noticeably. Persons of ordinary skilled in the art would understand that Ag content can vary depending on the application of the catalyst. Preferable content of Al₂O₃ of Ag/Al₂O₃ is 95 to 99 wt %.

The catalytic activity of the packed dual bed catalytic system also depends on the kind of metal deposited in ZSM-5 and the content of the metal.

A preferable metal deposited in ZSM-5 is copper (Cu). Preferably, the amount of Cu is from 0 to 5 wt %. More preferably, it is 2 to 5 wt %. Most preferably, it is about 4.66 wt %. DeNo_(x) activity does not increase significantly, if the Cu amount exceeds 5 wt %. It shall be evident that the zeolite ZSM-5 content varies from 95 to 100 wt %.

DeNO_(x) activity also depends on the ratio of Ag/Al₂O₃ to ZSM-5. A preferable ratio of Ag/Al₂O₃:ZSM-5 is about 4:1 to about 1:4 based on weight. A more preferable ratio is about 2:1. DeNO_(x) activity also depends on packing sequence of Ag/Al₂O₃ and ZSM-5. Preferably, Ag/Al₂O₃ is located at the front part of the system with ZSM-5 located at the rear part thereof.

In a further preferred embodiment, the present dual bed catalytic systems may comprise a hydrocarbon compound as a reductant of NO_(x). Preferably, the hydrocarbon compound may have 6-16 carbon atoms. More preferably, it is a C₁₀-C₁₆ alkane. Also preferably, it can be a mixture of hydrocarbon compounds. Diesel oil can be used as the reductant.

Practical and presently preferred embodiments of the present invention are illustrated as shown in the following examples. However, it will be appreciated that those skilled in the art may, in consideration of this disclosure, make modifications and improvements within the spirit and scope of the present invention.

EXAMPLE 1

A dual bed catalytic system comprising Ag/γ-Al₂O₃ and Cu/ZSM-5 catalyst was prepared as shown in Table 1 below. AgNO₃ was added as precursor to γ-Al₂O₃ (BET=204 m²/g) and impregnation was performed with an Ag content of 1, 2, 3 and 5 wt %. Cu/ZSM-5 was prepared by wet ion exchange. Three kinds of catalysts were prepared by varying Cu content. The physical and chemical properties of the catalysts are summarized in Table 1.

2 wt % Ag/γ-Al₂O₃ (front) and 4.66 wt % Cu/ZSM-5 (rear) were packed with the ratio of 2:1 based on weight to obtain a dual bed catalytic system.

EXPERIMENTAL EXAMPLE Measuring DeNO_(x) Activity

In order to measure the NO_(x) reducing efficiency of the dual bed catalytic system prepared in Example 1, the dual bed catalytic system was pretreated for 1 hour under the condition of 550° C., He balance and 10% O₂. Then, 1000 ppm NO, 10% O₂, 5% H₂O and 540 ppm n-dodecane (n-C₁₂H₂₆) as a stimulant of diesel oil and reductant were injected. The deNO_(x) reaction was performed at 200 to 500° C. at a space velocity of 30,000 h⁻¹. The conversion ratio of NO_(x) into N₂ was calculated. The result is given in FIG. 1.

On-line GC (HP 6890 series) was performed on a packed column (molecular sieve 5A) for the quantitative analysis of N₂.

EXAMPLE 2

The effect of the packing sequence of the dual bed catalytic system comprising Ag/γ-Al₂O₃ and Cu/ZSM-5 on the NO_(x) reducing efficiency was evaluated.

The mixture of 3.37 wt % Cu/ZSM-5 was packed at the front and 2 wt % Ag/γ-Al₂O₃ was packed at the rear side with the ratio of 1:1 based on weight. Then, 2 wt % Ag/γ-Al₂O₃ was packed at the front and 3.37 wt % Cu/ZSM-5 at the rear side with the ratio of 1:1 based on weight. Finally, 3.37 wt % Cu/ZSM-5 and 2 wt % Ag/γ-Al₂O₃ were physically mixed. NO_(x) reducing activity was measured. The result is given in FIG. 2.

EXAMPLE 3

The effect of the packing ratio of the dual bed catalytic system comprising Ag/γ-Al₂O₃ and Cu/ZSM-5 on the NO_(x) reducing efficiency was evaluated.

2 wt % Ag/γ-Al₂O₃ (front) and 3.37 wt % or 4.66 wt % Cu/ZSM-5 (rear) were packed with the ratio of 1:2 based on weight, 1:1 based on weight and 2:1 based on weight. The results are given in FIG. 3 and FIG. 4.

EXAMPLE 4

In order to find out the optimum Cu content of the dual bed catalytic system comprising Ag/γ-Al₂O₃ and ZSM-5, HZSM-5 with no Cu and Cu/ZSM-5 catalysts comprising 1.91 wt %, 3.37 wt % and 4.66 wt % of Cu were used to prepare dual bed catalytic systems.

2 wt % Ag/γ-Al₂O₃ (front) and ZSM-5 (rear) with different Cu content were packed with the ratio of 2:1 and NO_(x) reducing activity was measured. The result is given in FIG. 5.

EXAMPLE 5

In order to find out the optimum Ag content of the dual bed catalytic system comprising Ag/γ-Al₂O₃ and Cu/ZSM-5, silver (Ag) and Ag/γ-Al₂O₃ catalysts comprising 1, 2, 3 and 5 wt % of 4 silver were used to prepare dual bed catalytic systems.

Ag/γ-Al₂O₃ (front) and Cu/ZSM-5 (rear) were packed with the ratio of 2:1 based on weight of Ag/γ-Al₂O₃ to Cu/ZSM-5 based on weight. The result is given in FIG. 6.

COMPARATIVE EXAMPLES 1 TO 5

Catalytic systems were prepared using 2 wt % Ag/γ-Al₂O₃ and 3.37 wt % Cu/ZSM-5 as standard materials in the same manner as in Example 1. NO_(x) reducing efficiency was measured in the same manner as in Experimental Example 1. The result is given in FIGS. 1 to 6.

Also, three kinds of other dual bed catalytic systems comprising Ag/Y-Al₂O₃ and Cu/ZSM-5 were prepared.

First, Ag/γ-Al₂O₃ and Cu/Y were packed with the ratio of 2:1 based on weight. Second, Ag/γ-Al₂O₃ and Fe/ZSM-5 were packed with the ratio of 2:1 based on weight. At last, Ag/γ-Al₂O₃ and H/ZSM-5 were packed with the ratio of 2:1 based on weight.

Cu/Y was prepared by wet ion exchange. Fe/ZSM-5 was prepared by solid state ion exchange with FeCl₃ in the absence of moisture. Their physical and chemical properties are given in Table 1.

The NO_(x) reducing efficiency of the three types of dual bed catalytic systems are given in FIG. 1. The measurement was conducted in the same manner as in Experimental Example 1.

TABLE 1 Preparation Catalysts method Remarks   1 wt % Ag/γ-Al₂O₃ Impregnation γ-Al₂O₃ (BET = 204 m²/g) +   2 wt % Ag/γ-Al₂O₃ AgNO₃   3 wt % Ag/γ-Al₂O₃   5 wt % Ag/γ-Al₂O₃ HZSM-5 NH₄ ⁺/ZSM-5 (Si/Al = 14) 1.91 wt % Cu/ZSM-5 Wet ion NH₄ ⁺/ZSM-5 (Si/Al = 14) + 3.37 wt % Cu/ZSM-5 exchange Cu(acetate)₂ 4.66 wt % Cu/ZSM-5 8.17 wt % Cu/Y Wet ion HY (Si/Al = 2.5) + Cu(acetate)₂ exchange 1.96 wt % Fe/ZSM-5 Solid state NH₄ ⁺/ZSM-5 (Si/Al = 14) + FeCl₃ ion exchange

The NO_(x) reducing efficiency of the dual bed catalytic systems of Examples and Comparative Examples was compared.

As shown in FIG. 1, deNOx activity was evaluated for the dual bed catalytic systems comprising Cu/ZSM-5, Fe/ZSM-5, H/ZSM-5 or Cu/Y and 2 wt % Ag/γ-Al₂O₃. All other combinations showed lower activity than the dual bed catalytic system comprising 2 wt % Ag/γ-Al₂O₃ and 4.66 wt % Cu/ZSM-5.

The H/ZSM-5 catalyst showed improved catalytic activity at 300° C. compared with the single bed catalyst comprising 2 wt % Ag/γ-Al₂O₃. The CuY catalyst showed superior activity below 350° C. to the single bed catalyst comprising 2 wt % Ag/γ-Al₂O₃, but the activity decreased greatly over 350° C.

As shown in FIG. 2, NO_(x) reducing activity, which could not be found in the Ag/γ-Al₂O₃ catalyst, was observed at 300° C. without regard to the packing sequence. When the Ag/γ-Al₂O₃ was placed at the front side and the Cu/ZSM-5 was placed at the rear side, a significant improvement in activity was observed even at 350° C. or higher. The dual bed catalytic system comprising Fe/ZSM-5 showed superior deNO_(x) activity at all temperature to when only 2 wt % Ag/γ-Al₂O₃ was used.

As shown in FIGS. 3 and 4, deNO_(x) activity changed significantly depending on the packing ratio. Particularly, increase of deNO_(x) activity was observed at 350° C. or higher when Ag/γ-Al₂O₃ (front) and Cu/ZSM-5 (rear) was packed with the ratio of 2:1 based on weight. The activity was also best with the ratio of 2:1 based on weight when the Cu contents were different.

As shown in FIG. 5, the activity at low temperature varied with different Cu contents. The best deNO_(x) activity was observed at 300° C. when 4.66 wt % Cu/ZSM-5 was used. At 350° C. or higher, the effect of the Cu content was not important. The ZSM-5 catalyst which was not ion exchanged with Cu showed significantly improved catalytic activity at 300° C. than the single bed catalyst comprising 2 wt % Ag/γ-Al₂O₃.

As shown in FIG. 6, the activity was almost identical at 300° C., except when 1 wt % Ag/γ-Al₂O₃ was placed at the front. At 400° C. or above, the activity of the 5 wt % Ag/γ-Al₂O₃ catalyst dropped significantly. To conclude, a desirable effect can be attained when the silver (Ag) content is in the range of from 2 to 3 wt %.

As described above, the present dual bed catalytic systems provide excellent deNO_(x) activity, especially when using n-dodecane as a reductant than the single bed catalytic system comprising Ag/Al₂O₃. 40% or more improved activity can be attained at 300° C. at which little activity is observed when using Ag/Al₂O₃ only. Besides, a significantly improved activity is attained at 350° C. or above.

Thus, the present dual bed catalytic systems can be industrially applied for reducing nitrogen oxides emitted from a variety of non-moving and moving facilities, at 300° C. or above.

Preferred embodiments of the present invention have been described and illustrated, however, the present invention is not limited thereto, rather, it should be understood that various modifications and variations of the present invention can be made thereto by those skilled in the art without departing from the spirit and the technical scope of the present invention as defined by the appended claims. 

1. A dual bed catalytic system for the reduction of nitrogen oxides (NO_(x)), which comprises: alumina-supported silver (Ag/Al₂O₃) comprising 1-5 wt % of silver (Ag) and 95-99 wt % of alumina (Al₂O₃); and zeolite (ZSM-5) comprising 0-5 wt % of copper (Cu) and 95-100 wt % of zeolite (ZSM-5).
 2. The dual bed catalytic system as set forth in claim 1, wherein the Ag/Al₂O₃ and the ZSM-5 are packed.
 3. The dual bed catalytic system as set forth in claim 1, wherein the ZSM-5 is a copper (Cu) deposited ZSM-5.
 4. The dual bed catalytic system as set forth in claim 1, wherein ratio of the Ag/Al₂O₃ and the ZSM-5 is from 4:1 to 1:4 based on weight.
 5. The dual bed catalytic system as set forth in claim 1, wherein the Ag/Al₂O₃ is located at the front part of the system and the ZSM-5 is located at the rear part of the system.
 6. The dual bed catalytic system as set forth in claim 1, which comprises a C₆-C₁₆ hydrocarbon compound as a reductant of NO_(x). 